Fabrics comprising blends of cotton with polyester fibers are of considerable commercial importance because of their widespread use in apparel. However, such fabrics are highly flammable. To reduce the flammability of such blends, a flame retardant must be applied to the fabric. A flame retardant is a substance which increases the resistance of a substrate to burning or charring. Flame retardants are known for 100 percent cotton fabrics, but researchers in this field have concluded that the flammability of blend fabrics cannot be predicted from knowledge of the flammability of fabrics made from a single fiber, G. C. Tesoro, Status and Prospects for Flame Resistant Polyester/Cellulose Blend Fabrics, National Bureau of Standards Report COM-73-11265, March, 1973. One important reason for this unpredictability of blend fabrics, e.g., cotton/polyester blends, is the so-called "grid" or "scaffold" effect by which one component of the blend forms a supporting matrix for continued burning of the other component. With regard to this effect, W. Kruse reported:
"In all textile mixtures containing a component capable of forming a structural network (e.g. polyester) account must be taken of the scaffold effect, as it substantially alters the combustion behavior of thermoplastic synthetic fibers. PA0 "In mixed textiles the framework is built in all cases by organic material. Interestingly enough, it is also possible to arrive at a scaffold effect with inorganic material if, for instance, single component fabrics of thermoplastic fibers are given a framework forming finish (e.g. silicate)." W. Kruse, Melliand Textilber, April 1969, pp. 460-469 (Gottlieb Duttweiler Institute Publication No. 45, pp. 137-161, 1969), Combustibility and Flame Resistant Finishing of Mixed Textiles.
In accordance with existing technology, polyester fibers can be flame retarded. In one method, a brominated component, tetrabromobisphenol-A ethoxylate, is incorporated in the polymerization reaction producing the polyester from which the fiber is spun. In another method, a polyester fiber is treated topically with an aqueous dispersion of tris-(2,3-dibromopropyl)phosphate which, at elevated temperature, undergoes a thermally induced diffusion into the polyester fiber, thereby reducing the flammability of the fiber.
Polyester fibers having tetrabromobisphenol-A ethoxylate incorporated therein or treated with tris-(2,3-dibromopropyl)phosphate are themselves flame retardant, but when blended with cotton the fire retardance of the polyester is insufficient to render the blend flame retardant. In my co-pending application Ser. No. 571,738, now U.S. Pat. No. 3,997,699 there was disclosed a method of flame retarding cotton/polyester blend fabrics in which the polyester component was flame retarded with tetrabromobisphenol-A ethoxylate and then the blend fabric was treated with an aqueous solution of an organo phosphonic diamide, dried and cured to provide a material of significantly improved flame resistance. However, it is expensive and inconvenient to flame retard one component of a cotton/polyester blend before the blend fabric is produced. It would be more desirable to add both bromine and phosphorus in suitable form to the blend fabric as produced. This is the object of the present invention.